Ultraviolet (UV) radiation having a wavelength falling within the range of 290-400 nanometers (nm) has been associated with damage to human skin. UV-B radiation is produced between 290-320 nm and UV-A radiation between 320-400 nm. UV radiation damage to human skin may result in damage to the dermal infrastructure which, in its most extreme form may result in malignancies. UV radiation falling within the range of 290-320 nm may cause erythema and edema associated with sunburn.
Sunscreens which have been used in an attempt to block out harmful UV rays include organic sunscreens such as para-aminobenzoic acid (PABA). However, organic sunscreens do not block both UV-B and UV-A rays, and may have side-effects such as irritation and toxicity. Inorganic sunscreens offer broader protection in that they block radiation generally within the range of 290-366 nm. Inorganic sunscreens most often are made of particles of titanium dioxide or zinc oxide which lie on the skin surface. Since these particles are not absorbed by the skin, they usually do not produce skin irritation or toxic side effects.
UV radiation also damages surfaces other than human skin; for example, painted surfaces which are exposed to the sun. Zinc oxide or titanium dioxide have been used in paints to provide color and protection from damaging UV rays.
Prior art disclosures of metal oxides include preparations of spherical zinc or titanium oxide particles (U.S. Pat. No. 5,032,390, EP0 433 086 A1, U.S. Pat. Nos. 4,606,869, 3,397,257, 4,543,341, 4,808,398, 2,898,191, 4,9233,518, and 4,721,610), crystalline metal oxides (U.S. Pat. Nos. 5,093,099, 5,091,765, 4,261,965, 2,900,244, and 4,722,763), including crystalline whisker-shaped zinc oxide (U.S. Pat. No. 5,066,475), and needle-shaped zinc oxide particles (U.S. Pat. No. 5,102,650). U.S. Pat. No. 5,032,390 relates to a sunscreen composition containing from 1-25% by weight zinc oxide particles ranging in size from 70-300 .mu.m and having a spherical shape. U.S. Pat. No. 5,093,099 relates to a process for preparing flaky fine particles of zinc oxide for external use and having an average particle diameter of 0.1-1 micron, an average thickness of 0.01-0.2 micron, and an aspect ratio of at least 3. U.S. Pat. No. 5,066,475 relates to whiskers of zinc oxide having a crystal structure which includes a central body and four needle crystal projections radially extending therefrom, and is useful in reinforcing materials. U.S. Pat. No. 5,012,650 relates to needle-like electrically conductive zinc oxide filler which is useful for its low specific volume resistance and electrical conductivity.
It is an object of the invention to provide a composition for coating a surface which contains zinc oxide particles of dimensions which allow the particles to form a highly UV-protective coating. Zinc oxide particles of the invention, because they are rod-shaped, may assume a side-by-side arrangement or a criss-cross-packing arrangement, once the surface is coated with the composition, such that there are relatively few gaps between the particles for UV rays to penetrate. It is also an object of the invention to provide a UV protectant comprising zinc oxide particles which has a high ratio of surface area to volume or weight. It is another object of the invention to provide a composition for coating a surface which possesses even spreadability over the surface. It is another object of the invention to provide a composition containing zinc oxide particles for coating a surface which possesses a smooth texture and in which the particles are easily admixed with and dispersed within a spreadable vehicle. Further, it is an object of the invention to provide a highly UV protective surface coating which is transparent, white, or of a color other than white.